DRAM tunneling access transistor

ABSTRACT

In one embodiment, a first transistor is comprised of a first p+ source region doped in an n-well in the substrate and a first n+ drain region doped on one side at the top of the pillar. A second transistor is comprised of a second p+ source region doped into the second side of the top of the pillar and serially coupled to the top drain region for the first transistor. A second n+ drain region is doped into the substrate adjacent the pillar. Ultra-thin body layer run along each pillar sidewall between their respective active regions. A gate structure is formed along the pillar sidewalls and over the body layers. The transistors operate by electron tunneling from the source valence band to the gate bias-induced n-type channels, along the ultra-thin silicon bodies, thus resulting in a drain current.

RELATED APPLICATIONS

This Application is a Divisional of U.S. application Ser. No.11/219,085, titled “DRAM TUNNELING ACCESS TRANSISTOR”, filed Sep. 1,2005, now U.S. Pat. No. 7,446,372 (allowed) which is commonly assignedand incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to memory and in particular thepresent invention relates to dynamic random access memory.

BACKGROUND OF THE INVENTION

Transistor lengths have become so small that current continues to flowwhen they are turned off, draining batteries and affecting performance.When the gate-source voltage, V_(gs), of a metal oxide semiconductor(MOS) transistor is less than its voltage threshold, V_(t), it is in thesub-threshold region. This is characterized by a exponential change indrain current with V_(gs). Sub-threshold leakage currents are difficultto control and reduce in conventional nano-scale planar complementarymetal oxide semiconductor (CMOS) transistor technology. As technologyscales, sub-threshold leakage currents can grow exponentially and becomean increasingly large component of total power dissipation. This is ofgreat concern to designers of handheld or portable devices where batterylife is important, so minimizing power dissipation while achievingsatisfactory performance is an increasingly important goal.

Two-dimensional short channel effects in a typical prior art planartransistor structure, shown in FIG. 1, result in a sub-threshold slopeon the order of 120 mV/decade to 80 mV/decade. An ideal slope would beapproximately 60 mV/decade, as shown in FIG. 2. The low power supplyvoltages used in nano-scale CMOS circuits that are now on the order of2.5 V exacerbate the problem.

The planar transistor of FIG. 1 is comprised of a substrate 100 in whichtwo source/drain regions 101, 102 are implanted. A control gate 103 isformed over the channel region 105 in which a channel forms duringoperation of the transistor.

Future supply voltages are projected to become even lower, in the rangeof 1.2 V, as designers try to improve battery life and performance ofelectronic devices. At such power levels, there will not be enoughvoltage range to turn on a transistor. A significant voltage overdriveabove the threshold voltage is typically required to turn-on a prior arttransistor and turn-off the transistor sub-threshold leakage. This canbe several multiples of the 100 mV/decade threshold voltage slopeillustrated in FIG. 2. For good I_(on)/I_(off) ratios, the sub-thresholdleakage current needs to be at least eight orders of magnitude or eightdecades below the transistor current levels when the transistor isturned on. With a 1.2 V voltage range, there will not be enough voltageswing to allow both objectives: high on current and low sub-thresholdleakage to be accomplished with conventional planar devices.

Gate body connected transistors as previously described in CMOS circuitsprovide a dynamic or changing threshold voltage, low when the transistoris on and a high threshold when it is off. Another alternative is usingdual gated transistors. Yet another alternative is surrounding gatestructures where the gate completely surrounds the transistor channel.This allows best control over the transistor channel but the structurehas been difficult to realize in practice. Another technique has been tore-crystallize amorphous silicon that passes through a horizontal orvertical hole. None of these techniques, however, can have asub-threshold slope less than the ideal characteristic of 60 mV/decadefor a convention MOSFET.

For the reasons stated above, and for other reasons stated below thatwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art fora device structure that has reduced sub-threshold leakage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a typical prior art planar CMOStransistor structure.

FIG. 2 shows a graphical plot of sub-threshold leakage current for atypical prior art CMOS transistor as compared to an ideal sub-thresholdleakage characteristic.

FIG. 3 shows a schematic cross-sectional view of two ultra-thin siliconbody tunneling transistors of the present invention.

FIG. 4 shows a circuit symbol in accordance with a first of thetunneling transistors of the embodiment of FIG. 3.

FIG. 5 shows a circuit symbol in accordance with a second of thetunneling transistors of the embodiment of FIG. 3.

FIGS. 6A and 6B show energy band diagrams of the electrical operation ofthe tunneling transistor embodiment of FIG. 3.

FIG. 7 shows a plot of the sub-threshold leakage current of thetunneling transistor embodiment of FIG. 3.

FIG. 8 shows fabrication process steps in accordance with the twoultra-thin silicon body tunneling transistors of the present invention.

FIG. 9 shows additional fabrication process steps in accordance with thetwo ultra-thin silicon body tunneling transistors of the presentinvention.

FIG. 10 shows a top cross-section view of one embodiment of the twoultra-thin silicon body tunneling transistors of the present inventionalong axis A-A′ of FIG. 9.

FIG. 11 shows a schematic diagram of one application of the embodimentsof the ultra-thin silicon body tunneling transistors of the presentinvention as DRAM access transistors.

FIG. 12 shows a schematic diagram of an open bit DRAM array structureapplication using the ultra-thin silicon body tunneling transistors ofthe present invention.

FIG. 13 shows a block diagram of one embodiment of a memory deviceincorporating the embodiments of the vertical tunneling, ultra-thin bodytransistor of the present invention.

FIG. 14 shows a block diagram of one embodiment of a memory moduleincorporating the embodiments of the vertical tunneling, ultra-thin bodytransistor of the present invention.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference ismade to the accompanying drawings that form a part hereof and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims and equivalents thereof. The terms wafer or substrate used in thefollowing description include any base semiconductor structure. Both areto be understood as including silicon-on-sapphire (SOS) technology,silicon-on-insulator (SOI) technology, thin film transistor (TFT)technology, doped and undoped semiconductors, epitaxial layers of asilicon supported by a base semiconductor structure, as well as othersemiconductor structures well known to one skilled in the art.Furthermore, when reference is made to a wafer or substrate in thefollowing description, previous process steps may have been utilized toform regions/junctions in the base semiconductor structure, and termswafer or substrate include the underlying layers containing suchregions/junctions.

FIG. 3 illustrates schematic cross-sectional view of one embodiment fortwo ultra-thin silicon body, tunneling NMOS transistors 350, 351 of thepresent invention. For purposes of clarity, FIG. 3 shows the twotransistors 350, 351 as being separated. However, as is shown anddiscussed subsequently, the transistors 350, 351 are formed around thesame oxide pillar 330. In fact, the drain 311 of the first transistor isseries connected to the source 310 of the second transistor over theoxide pillar 330.

The illustrated embodiment is formed in a p-type silicon substrate 360and a doped n-well 300 in the substrate 360. Alternate embodiments mayuse other conductivity doping for the substrate/well and/or othermaterials for the substrate instead of silicon.

Instead of the conventional n+ source region formed in the n-well 300,the source 301 of the left most transistor 351 is p+ doped.Additionally, the source wiring that couples the source to othercomponents in a circuit is also p+ doped. The drain 302 of the righttransistor 350 is an n+ region doped in the substrate 360.

An oxide pillar 330 is formed over the n-well 300 and substrate 360.Ultra-thin, lightly doped, p-type body layers 368, 369 are formed alongthe sides of the oxide pillar 330. In one embodiment, the dualtransistors are implemented in 0.1 micron technology such that thetransistor has a height of approximately 100 nm and a thickness in therange of 25 to 50 nm. The p-type body layers 368, 369 have a thicknessin the range of 5 to 20 nm. Alternate embodiments may use otherdimensions. Alternate embodiments can have other heights and/orthickness ranges.

The left transistor 351 has an n+ doped drain region 311 formed at thetop of the left silicon body 369 and oxide pillar 330. The righttransistor 350 has a p+ doped source region 310 formed at the top of theright silicon body 368 and oxide pillar 330.

A gate insulator layer 305, 306 is formed over each ultra-thin siliconbody 368, 369. The insulator can be an oxide or some other type ofdielectric material.

A gate structure 303, 304 is formed over each insulator layer 305, 306.In one embodiment, the gate is comprised of polysilicon. As is wellknown in the art, proper biasing of the gates 303, 304 induce ann-channel 307, 308 to form in a channel region between their respectivesource 310, 301 and drain 302, 311 regions.

The electrical operation of the transistors is based on a MOS-gatedpin-diode. During operation of the embodiment illustrated in FIG. 3, thegates 304, 303 are biased to induce n-type channels 321, 322 to form inthe ultra-thin bodies 369, 368. A drain 311 bias causes tunneling tooccur from the source 301 valence band to the n-channel 321 resulting ina drain current in the left transistor. The drain-to-source current(I_(DS)) of the left transistor 351 flows from the top drain region 311to the bottom source region 301. I_(DS) of the right transistor 350flows from the bottom drain region 302 to the top source region 310.

FIGS. 4 and 5 illustrate circuit diagram symbols of the ultra-thin body,tunneling transistors of the embodiment of FIG. 3. FIG. 4 illustratesthe left transistor 351 of FIG. 3. FIG. 5 illustrates the righttransistor 350 of FIG. 3.

FIGS. 6A and 6B illustrate energy band diagrams of the operation of thetransistor of FIG. 3. The upper line of each figure indicating theenergy of the conduction band and the lower line indicating the energyof the valence band. FIG. 6A illustrates a no bias condition for thetransistor. The diagram shows the channel and n+ drain 601 and p+ source602. In the non-conducting condition, a large barrier 603 exists betweenthe drain 601 and source 602 regions.

FIG. 6B illustrates that applying a bias to the gate creates aconducting condition in which an electron channel is induced to formwhere the electron concentration is degenerated. A tunnel junction 605is formed at the source side 602 of the channel.

Applying a drain bias causes band bending and the n-type regionconduction band to be below the valence band edge in the source region.Electrons can then tunnel from the source to the n-channel regions.Since there can be no tunneling until the conduction band edge in thechannel is drawn below the valence band in the source, the turn-oncharacteristic is very sharp and the sub-threshold slope approaches theideal value for a tunneling transistor of zero mV/decade as illustratedin FIG. 7.

FIG. 7 illustrates a plot of drain current versus the gate-to-sourcevoltage (V_(GS)) of the transistor. This plot shows the very steepsub-threshold slope “S” 701 that results from the biasing of theembodiments of the ultra-thin body transistor of the present invention.The vertical, drain current axis of FIG. 7 is a log scale while thehorizontal, V_(GS) axis is linear.

FIG. 8 illustrates one embodiment of a method for fabricating thevertical tunneling, ultra-thin silicon body transistors of the presentinvention. In this embodiment, oxide pillars 801 are formed by an etchprocess on the surface of a substrate 800. In one embodiment, thesubstrate/well 800 is a p-type silicon. Amorphous silicon 802 isre-crystallized over the substrate 800 surface and oxide pillars 801.This can be accomplished by solid phase epitaxial growth.

Since crystal growth can occur over short distances, the top of thepillar 801 can have grain boundaries 803 in the polycrystalline silicon802. As is well known in the art, a grain boundary is the boundarybetween grains in polycrystalline material. It is a discontinuity of thematerial structure having an effect on its fundamental properties.

FIG. 9 illustrates further fabrication steps for the transistorembodiments of the present invention. The sidewalls of the pillar 901are the ultra-thin bodies 903, 923 that are lightly doped p-typesilicon. The wafers are masked and the n-wells are implanted. The p-typeregions 921, 925 are implanted without using a mask. The pillars aremasked and the unmasked drain regions 924 and 902 are implanted n+.Since the p+ doping is always lower than the n+, the n+ regions can beimplanted over the p+ regions and they will be n+.

A gate insulator layer 904 is grown or deposited over the silicon layers903, 923. In one embodiment, the gate insulator layer 904 is an oxide.The gates 906, 922 are formed over the insulator 904. In one embodiment,the gates 906, 922 are formed by a sidewall etch technique. A datacapacitor contact 910 is added to the top of the pillar 901 to enableconnection of the series connect node between the drain 924 of the firsttransistor and the source 925 of the second transistor. A cross-sectionalong axis A-A′ of FIG. 9 is illustrated in FIG. 10 to show thestructure of the transistors of the present invention.

FIG. 10 illustrates a top view of cross-section A-A′ of FIG. 9 of a dualgated embodiment of the two vertical tunneling, ultra-thin bodytransistors of the present invention. This view shows the two gates1001, 1002 formed around the deposited oxide 1004 and as a gateinsulator 1005, 1006. Deposited oxide 1004 also separates adjacenttransistor pillars. The ultra-thin bodies 1010, 1011 are located oneither side of the oxide pillar 1013.

FIG. 11 illustrates one embodiment of an application of the verticaltunneling, ultra-thin body transistors as DRAM access transistors. Thisfigure shows a DRAM cell circuit 1150 using the transistors of thepresent invention to control the coupling of the DRAM data capacitors1106, 1107 to the sense amplifier 1109.

This figure shows two sets 1100, 1101 of transistors as describedpreviously. The read address 1120, 1121 and read data bit lines 1140,1141 are separate and apart from the write address lines 1130, 1131 andwrite data bit lines 1143, 1144. This is necessary as the tunneling NMOStransistors 1100, 1101 of the present invention are not symmetrical likeconventional prior art NMOS transistors. The tunneling transistors 1100,1101 conduct current in only one direction, into the n+ drain and out ofthe p+ source. The NMOS transistor on one side of the pillar 1110, 1111reads data while the NMOS transistor on the other side of the pillar1112, 1113 writes data into the storage capacitor 1106, 1107. The senseamplifier 1109 senses the current on the data/bit lines to determine thestate of the data capacitor 1106, 1107.

FIG. 12 illustrates another embodiment of an application of the verticaltunneling ultra-thin body transistors as DRAM access transistors inaccordance with the embodiment of FIG. 11. This figure shows how thecircuit 1150 of FIG. 11 fits in an open bit line DRAM array withseparate write address and read address lines as well as separate readdata/bit and write data/bit lines.

The embodiment of FIG. 12 is comprised of memory cell arrays 1210, 1211that are each coupled to a row address decode circuit 1202, 1203 and acolumn address decode circuit 1205, 1206. The column of sense amplifiers1201 senses the state of each row of DRAM cells.

FIG. 13 illustrates a functional block diagram of a memory device 1300of one embodiment of the present invention. The memory device 1300 isanother embodiment of a circuit that can include the ultra-thin bodyaccess transistors of the present invention.

The memory device includes an array of memory cells 1330 such as DRAMtype memory cells or non-volatile memory cells. The memory array 1330 isarranged in banks of rows and columns along word lines and bit lines,respectively.

An address buffer circuit 1340 is provided to latch address signalsprovided on address input connections A0-Ax 1342. Address signals arereceived and decoded by a row decoder 1344 and a column decoder 1346 toaccess the memory array 1330. It will be appreciated by those skilled inthe art, with the benefit of the present description, that the number ofaddress input connections depends on the density and architecture of thememory array 1330. That is, the number of addresses increases with bothincreased memory cell counts and increased bank and block counts.

The memory device 1300 reads data in the memory array 1330 by sensingvoltage or current changes in the memory array columns using sense/latchcircuitry 1350. The sense/latch circuitry, in one embodiment, is coupledto read and latch a row of data from the memory array 1330. Data inputand output buffer circuitry 1360 is included for bi-directional datacommunication over a plurality of data connections 1362 with thecontroller 1310). Write circuitry 1355 is provided to write data to thememory array.

Control circuitry 1370 decodes signals provided on control connections1372 from the processor 1310. These signals are used to control theoperations on the memory array 1330, including data read, data write,and erase operations. The control circuitry 1370 may be a state machine,a sequencer, or some other type of controller.

The memory device illustrated in FIG. 13 has been simplified tofacilitate a basic understanding of the features of the memory. A moredetailed understanding of internal circuitry and functions of DRAM'sand/or flash memories are known to those skilled in the art.

The vertical tunneling, ultra-thin body transistors of the presentinvention can be used in the memory device of FIG. 13, as well as thesubsequently discussed memory module, as select transistors, controltransistors, and in logic elements such as NAND and NOR gates.

FIG. 14 is an illustration of an exemplary memory module 1400. Memorymodule 1400 is illustrated as a memory card, although the conceptsdiscussed with reference to memory module 1400 are applicable to othertypes of removable or portable memory, e.g., USB flash drives, and areintended to be within the scope of “memory module” as used herein. Inaddition, although one example form factor is depicted in FIG. 14, theseconcepts are applicable to other form factors as well.

In some embodiments, memory module 1400 will include a housing 1405 (asdepicted) to enclose one or more memory devices 1410, though such ahousing is not essential to all devices or device applications. At leastone memory device 1410 is a non-volatile memory [including or adapted toperform elements of the invention]. Where present, the housing 1405includes one or more contacts 1415 for communication with a host device.Examples of host devices include digital cameras, digital recording andplayback devices, PDAs, personal computers, memory card readers,interface hubs and the like. For some embodiments, the contacts 1415 arein the form of a standardized interface. For example, with a USB flashdrive, the contacts 1415 might be in the form of a USB Type-A maleconnector. For some embodiments, the contacts 1415 are in the form of asemi-proprietary interface, such as might be found on CompactFlash™memory cards licensed by SanDisk Corporation, Memory Stick™ memory cardslicensed by Sony Corporation, SD Secure Digital™ memory cards licensedby Toshiba Corporation and the like. In general, however, contacts 1415provide an interface for passing control, address and/or data signalsbetween the memory module 1400 and a host having compatible receptorsfor the contacts 1415.

The memory module 1400 may optionally include additional circuitry 1420which may be one or more integrated circuits and/or discrete components.For some embodiments, the additional circuitry 1420 may include a memorycontroller for controlling access across multiple memory devices 1410and/or for providing a translation layer between an external host and amemory device 1410. For example, there may not be a one-to-onecorrespondence between the number of contacts 1415 and a number of I/Oconnections to the one or more memory devices 1410. Thus, a memorycontroller could selectively couple an I/O connection (not shown in FIG.14) of a memory device 1410 to receive the appropriate signal at theappropriate I/O connection at the appropriate time or to provide theappropriate signal at the appropriate contact 1415 at the appropriatetime. Similarly, the communication protocol between a host and thememory module 1400 may be different than what is required for access ofa memory device 1410. A memory controller could then translate thecommand sequences received from a host into the appropriate commandsequences to achieve the desired access to the memory device 1410. Suchtranslation may further include changes in signal voltage levels inaddition to command sequences.

The additional circuitry 1420 may further include functionalityunrelated to control of a memory device 1410 such as logic functions asmight be performed by an ASIC (application specific integrated circuit).Also, the additional circuitry 1420 may include circuitry to restrictread or write access to the memory module 1400, such as passwordprotection, biometrics or the like. The additional circuitry 1420 mayinclude circuitry to indicate a status of the memory module 1400. Forexample, the additional circuitry 1420 may include functionality todetermine whether power is being supplied to the memory module 1400 andwhether the memory module 1400 is currently being accessed, and todisplay an indication of its status, such as a solid light while poweredand a flashing light while being accessed. The additional circuitry 1420may further include passive devices, such as decoupling capacitors tohelp regulate power requirements within the memory module 1400.

CONCLUSION

In summary, a vertical tunneling, ultra-thin body transistor NMOS FEThas a p+ source, rather than an n+ source as in prior art transistors.In this configuration, electrons tunnel from the p+ source to inducedn-channels along the ultra-thin body sidewalls of an oxide pillar. Sucha configuration provides an ideal sub-threshold slope that issubstantially close to 0 mV/decade and thus obtain low sub-thresholdleakage current in CMOS circuits. The substantially reduced leakagecurrent reduces the power requirements for electronic circuits.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe invention will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the invention. It is manifestly intended that thisinvention be limited only by the following claims and equivalentsthereof.

1. A method for fabricating a pair of vertical tunneling, ultra-thinbody transistors on a substrate, the method comprising: forming an oxidepillar on the substrate surface; forming an amorphous silicon layer overthe substrate surface and the pillar such that the amorphous siliconlayer, on opposing sides of the pillar, forms ultra-thin silicon bodies;doping a p+ region into the substrate on a first side of the pillar anda first portion of the amorphous silicon layer on top of a second sideof the pillar; doping an n+ region into a second portion of theamorphous silicon layer on top of the first side of the pillar and aregion of the substrate on the second side of the pillar; forming ann-well in the substrate on the first side of the pillar such that the p+region in the substrate resides in the n-well and the n+ region on thesecond side of the pillar does not reside in the n-well; forming a gateinsulator over the amorphous silicon layer; and forming a gate structureover the gate insulator on each side of the pillar.
 2. The method ofclaim 1 wherein the doping is performed by ion implantation.
 3. Themethod of claim 1 wherein the amorphous silicon layer is formed byregrowth of crystalline silicon along sidewalls of the oxide pillar bysolid phase epitaxial growth.
 4. The method of claim 1 wherein formingthe gate structure comprises forming a polysilicon gate.
 5. The methodof claim 1 wherein doping the p+ regions includes creating p+ sourceregions in the substrate n-well for a first transistor of the pair andthe top of the pillar for the second transistor such that verticaltunneling from the source regions through their respective siliconbodies occurs in response to biasing of the gate structures and the n+regions.
 6. A method for fabricating a pair of vertical tunneling accesstransistors with a DRAM cell in a substrate, the method comprising:forming an oxide pillar on the substrate surface; forming an amorphoussilicon layer over the surface and the pillar forming ultra-thin siliconbodies along sidewalls of the pillar; doping portions of the amorphoussilicon layer on the top of the pillar to form an n+ drain region and ap+ source region coupled together serially at a node; doping portions ofthe amorphous silicon layer adjacent to the pillars and the substrate toform an n-well in the substrate, a p+ source region in the n-well, andan n+ drain region in the substrate; forming an insulator layer overeach silicon body to act as a gate insulator; and forming separate gatestructures on opposing sides of the pillar over the silicon bodies; andforming a capacitor coupled to the node.
 7. The method of claim 6wherein at least one of the silicon bodies is formed to a thickness in arange of 5-20 nm, the pair of transistors is formed to a height of 100nm and a thickness in a range of 25-50 nm.
 8. The method of claim 6wherein the gate structure is formed from polysilicon.
 9. The method ofclaim 6 wherein the pillar is formed to a height of 100 nm.
 10. Themethod of claim 6 wherein the gate insulator is comprised of an oxide.11. The method of claim 6 and further including forming p+ wires intothe substrate n-well and coupled to the p+ source region.
 12. The methodof claim 6 wherein the gate insulator is either grown or deposited.